Formation fluids, including oil and gas produced at a well head are typically conveyed through flow lines to remote processing equipment. It is conventional practice to use surface safety valves which are responsive to changes in operating conditions to automatically shut off flow in the flow lines at the onset of unusual or unscheduled operating conditions. Such surface safety valve installations are designed to automatically close in response to fluctuations in selected conditions in the flow lines, either above or below predetermined settings, such as high and low liquid levels, high and low temperatures, high and low pressures and the like.
Conventional surface safety valves typically include a pneumatic or hydraulic actuator coupled to a gate valve for selectively permitting or disallowing flow of production fluids through the flow lines. For example, surface safety valves may be installed as a secondary master valve on a well head tree or as a wing valve directly on the flow line. Surface safety valves typically include a valve body having a central axis aligned with inlet and outlet passages and a space therebetween to receive the gate that may be moved perpendicularly to open and close the valve. In the closed position, the gate surfaces typically seal against sealing rings which surround the fluid passage through the valve body.
One type of surface safety valve includes a pneumatic actuator that is operated by a pneumatic supply system that is independent of well fluids and pressures. The pneumatic surface safety valve is designed to be held open by pneumatic control pressure acting on an actuator piston. Loss of pneumatic pressure in the actuator cylinder permits the well or flow line pressure acting on the gate along with the force exerted by a closing spring to drive the gate into a closed position. Such an actuator may be termed "fail safe," since in the event of an emergency causing loss of pneumatic pressure, the actuator will automatically cause the valve to assume the safe or closed state.
It has been found, however, that the use of pneumatic controlled actuators for surface safety valves is limited due to the size requirements of the actuator piston needed to operate gate valves particularly for high pressure and high volume flow lines which may require large bore gate valves. In addition, due to condensation and contamination within the air system utilized for pneumatic actuation, it has been found that venting of the air into the atmosphere is environmentally unsatisfactory.
To overcome the size limitation of pneumatic controlled actuators, another type of surface safety valve utilizes a hydraulic actuator that employs a hydraulic circuit to operate the actuator and to open and close the surface safety valve. The hydraulic actuators are typically part of a large hydraulic system that is controlled by a remote hydraulic control panel. As with the pneumatic actuators, the hydraulic actuators typically operate by acting hydraulic control pressure on an actuator piston. It has been found, however, that the gate and actuator piston in a hydraulic system will stroke at a limited speed due to the flow rate of hydraulic fluid and the volume of hydraulic fluid that is typically used in hydraulic systems.
Along with the surface safety valve on the well head, it is common for producing wells to include a subsurface safety valve located in the well production tubing several hundred feet below the ground surface. Subsurface safety valves may typically be flapper valves or ball valves which may be carried in a tubing connection or may be installed and set in place by wireline. Subsurface safety valves are typically operated using hydraulic fluid to operate the actuator to an open position. As with hydraulic actuated surface safety valves, when an out of range condition occurs, hydraulic pressure is released and the subsurface safety valve will actuate to the valve closed position. It has been found, however, that as with hydraulic actuated surface safety valves, the volume of hydraulic fluid in the hydraulic system affects the speed and depth at which the subsurface safety valves will operate.
Therefore, a need has arisen for a control system for actuating surface safety valves and subsurface safety valves that minimizes the volume of hydraulic fluid necessary to operate a hydraulic actuator between the valve closed position and the valve open position. A need has also arisen for such a control system that eliminates the need for a remote hydraulic control panel through which hydraulic fluid is circulated to multiple hydraulically controllable devices. Further, a need has arisen for such a control system that may be attached to existing surface safety valve actuator and subsurface safety valve actuators.